1. Field of the Invention
The present invention relates to acoustic wave filter devices, such as surface acoustic wave filter devices and boundary acoustic wave filter devices. More specifically, the present invention relates to an acoustic wave filter device in which a plurality of inductors are connected between an input terminal and an output terminal and in which a plurality of resonators are connected between a series arm coupling the input terminal and output terminal and the ground potential.
2. Description of the Related Art
Conventionally, various surface acoustic wave filter devices are used as trap filters in RF stages of communication apparatuses, such as mobile phones. For example, Japanese Unexamined Patent Application Publication No. 2004-129238 discloses a trap filter including a plurality of inductances serially connected between an input terminal and an output terminal and parallel arm resonators each connected between the input terminal and the ground potential, between the output terminal and the ground potential, and between a node of adjacent inductances and the ground potential.
FIG. 12 is a circuit diagram illustrating a circuit configuration of the trap filter mentioned above. In a trap filter 501, an inductance L1 is connected between an input terminal IN and an output terminal OUT. A parallel arm resonator P1 is connected between the input terminal IN and a ground potential. In addition, a parallel arm resonator P2 is connected between the output terminal OUT and the ground potential. In this filter, each of the parallel arm resonators P1 and P2 is defined by a surface acoustic wave resonator in which an IDT electrode is disposed on a piezoelectric substrate. In addition, the resonant frequencies of the parallel arm resonators P1 and P2 are set to be substantially equal and located in a stop band.
On the other hand, Japanese Unexamined Patent Application Publication No. 2003-332881 discloses a surface acoustic wave filter device in which first one-terminal-pair surface acoustic wave resonators having a predetermined anti-resonant frequency are connected to a series arm coupling an input terminal and an output terminal, and a second one-terminal-pair surface acoustic wave resonator is connected between first ends of the first one-terminal-pair boundary acoustic wave resonators and a ground potential. In this filter, the resonant frequency of the second one-terminal-pair surface acoustic wave resonator is set to be substantially equal to the anti-resonant frequency of the first one-terminal-pair surface acoustic wave resonators, and as a result, a trap providing a large amount of attenuation is formed.
In the trap filter 501 disclosed in Japanese Unexamined Patent Application Publication No. 2004-129238, the inductance L1 and the parallel arm resonators P1 and P2 define a stop band, i.e., a trap. However, the attenuation characteristic in a frequency band lower than the trap is flat, and thus, characteristics as a band-pass filter cannot be obtained on the lower frequency side.
Also in the surface acoustic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 2003-332881, a trap is provided by setting the anti-resonant frequency of the first one-terminal-pair surface acoustic wave resonators and the resonant frequency of the second one-terminal-pair surface acoustic wave resonator to be substantially equal, as described above. In this case, a characteristic as a trap filter is obtained by series resonance of the inductors serially connected with the first one-terminal-pair surface acoustic wave resonators and capacitance of the one-terminal-pair surface acoustic wave resonators. However, in the characteristic as a trap filter, the width of the passband is relatively small on the frequency side lower than the trap.
On the other hand, in mobile phones equipped with recording functions of ground-wave digital television DVB-H, a pass band with a sufficiently broad band width on the frequency side lower than the trap is required in order to enable recording during transmission. However, with the trap filters disclosed in Japanese Unexamined Patent Application Publication No. 2004-129238 and in Japanese Unexamined Patent Application Publication No. 2003-332881, as described above, it is difficult to provide a pass band having a sufficient band width on the frequency side lower than the trap.